In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
For example, multi-antenna techniques can potentially increase the performance in terms of data rates and reliability of a wireless communications network. The performance can, in particular, be improved if both the transmitting device and the receiving device are equipped with multiple antennas, which results in a multiple-input multiple-output (MIMO) communication channel. Such networks and/or related techniques are commonly referred to as MIMO.
The Long Term Evolution (LTE) standard is currently evolving to support MIMO, such as MIMO antenna deployments and MIMO related techniques. Currently, the so-called LTE-Advanced standard supports an 8-layer spatial multiplexing mode for 8 transmitting antennas with channel dependent precoding. The spatial multiplexing mode is aimed for high data rates in favorable channel conditions. An illustration of the spatial multiplexing operation is provided in FIG. 2. FIG. 2 schematically illustrates a network node 150 comprising an example transmission structure for the precoded spatial multiplexing mode in LTE. As seen in FIG. 2, an information carrying symbol vector s from r input layers (denoted Layer 1, Layer 2, . . . Layer r) is multiplied by an NT times r precoder matrix W. This precoder matrix serves to distribute the transmit energy in a subspace of the NT-dimensional vector space (corresponding to NT antenna ports), where the thus precoded symbol vector is fed to Inverse Fast Fourier Transformers before being fed to the antenna ports and then to an antenna array of a radio access network node. The precoder matrix is typically selected from a codebook of possible precoder matrices, and typically indicated by means of a precoder matrix indicator (PMI). The PMI specifies a unique precoder matrix in the codebook for a given number of symbol streams. The r symbols in s each correspond to a layer and r is referred to as the transmission rank. In this way, spatial multiplexing is achieved since multiple symbols can be simultaneously transmitted over the same time/frequency resource element (TFRE). The number of symbols r is typically adapted to suit the current channel properties.
In the LTE standard, orthogonal frequency-division multiplexing (OFDM) is used in the downlink (i.e. for transmission from radio access network nodes to wireless devices served by the network) and Discrete Fourier Transform (DFT) precoded OFDM is used in the uplink (i.e. for transmission to radio access network nodes from wireless devices served by the network) and hence the received NR×1 vector yn for a certain TFRE on a subcarrier n (or alternatively data TFRE number n) is modeled byyn=HnWsnnn  Equation 1where nn is a noise/interference vector obtained as realizations of a random process. The precoder W can be a wideband precoder which is constant over frequency, or a frequency selective precoder.
The precoder matrix W can be chosen to match the characteristics of the NR×NT dimensional MIMO channel matrix Hn, resulting in so-called channel dependent precoding. This is also commonly referred to as closed-loop precoding and essentially strives for focusing the transmit energy into a subspace which is strong in the sense of conveying much of the transmitted energy to one of the served wireless devices. In addition, the precoder matrix may also be selected to strive for orthogonalizing the channel, meaning that after proper linear equalization at the served wireless device, the inter-layer interference is reduced.
However, there is still a need for improved mechanisms for flexible selection of precoder matrix.